1. Field of the Invention
The present invention relates to a method for producing chlorinated vinyl chloride resin by chlorinating vinyl chloride or a copolymer resin thereof.
2. Description of Related Art
Chlorinated vinyl chloride resin (hereinafter, referred to as “CPVC”) is one of the general-purpose resins having excellent heat resistance. The heat resistance is more advantageous as the amount of chlorine (chlorination degree) contained in the molecules of the CPVC is larger. As a method for producing the CPVC, the following method generally is used: vinyl chloride resin (hereinafter, referred to as “PVC”) is suspended in an aqueous medium, and gaseous or liquid chlorine is supplied to the suspension, whereby the PVC is chlorinated. Furthermore, regarding such chlorination, photochlorination using light (UV-light) and thermochlorination using heat are performed industrially (JP 46(1971)-17128 B).
It is considered that the chlorination reaction is composed of the following three processes:
(1) Cl2→2Cl. (radical generation reaction)
(2) PVC+Cl.→PVC.+HCl (hydrogen abstraction reaction)
(3) PVC+Cl.→CPVC (chlorination reaction)
The process (1) is a radical generation reaction of generating a chlorine radical from chlorine with light or heat. The process (2) is a hydrogen abstraction reaction of abstracting hydrogen from PVC with the chlorine radical. The process (3) is a chlorination reaction of generating CPVC with a polymer radical (PVC.) and the chlorine radical or chlorine. The processes (1) to (3) are represented collectively by the following reaction formula (A):PVC+Cl2→CPVC+HCl   (A)
Thus, in the chlorination reaction as shown in the process (A), only about one half of the chlorine material is used for a chlorination reaction, and the other half thereof is consumed for by-producing hydrochloric acid. Therefore, the concentration of hydrochloric acid in a slurry after the chlorination reaction becomes, for example, 5 to 12% by weight with respect to the water in the slurry, with the result that the corrosion of the equipment such as a reactor and a drier becomes remarkable. This makes it necessary to coat the equipment with expensive metal so as to prevent the corrosion of the equipment. Accordingly, there arises an industrial problem that a large amount of water is required for cleaning so as to reduce the problem of removing hydrochloric acid in the drier.
For the above-mentioned purpose, it is advantageous in terms of cost to adopt a process of cleaning out and removing even hydrochloric acid in resin with hot water. However, even with such a process, the amount of hydrochloric acid before drying is about 1,000 ppm, and in order to reduce the problem of removing hydrochloric acid in the drying process, the concentration of hydrochloric acid before drying needs to be set to be 100 ppm or less.
Various attempts have been made so as to reduce the problem of removing hydrochloric acid. For example, a method for using citric acid or various kinds of salts thereof so as to neutralize by-produced hydrochloric acid has been proposed (U.S. Pat. No. 5,359,011). However, according to this method, citric acid or various kinds of salts thereof become impurities, so that a great cost is incurred for using and cleaning them, and in addition, it is difficult to set the remaining amount of hydrochloric acid to be 100 ppm or less.
Furthermore, a method for removing hydrochloric acid using a filter also has been proposed (JP 2003-238615 A). However, even using this method, it is difficult to set the remaining amount of hydrochloric acid to be 100 ppm or less.
Furthermore, a method for generating chlorine with proton acid and hypochlorite, instead of blowing chlorine, thereby performing chlorination (JP 2001-11116 A) or a method for placing hypochlorite during a chlorination reaction with chlorine (JP 2004-99669 A) also has been proposed. Although these methods are capable of reducing the concentration of hydrochloric acid in water, it is difficult to set the remaining amount of hydrochloric acid in resin to be 100 ppm or less.
Thus, in the conventional production of CPVC, there are various industrial problems such as the problem of equipment corrosion caused by hydrochloric acid in a slurry, and the problem of decrease in initial coloring and transparency, and the development of a technique of enhancing the balance thereof has been a long-term subject for those skilled in the art.